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/*
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 * Copyright (c) 2009-2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  * Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
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 *  * Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
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 *    may be used to endorse or promote products derived from this software
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package java.time.zone;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.NavigableMap;
import java.util.Objects;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * Provider of time-zone rules to the system.
 * <p>
 * This class manages the configuration of time-zone rules.
 * The static methods provide the public API that can be used to manage the providers.
 * The abstract methods provide the SPI that allows rules to be provided.
 * <p>
 * ZoneRulesProvider may be installed in an instance of the Java Platform as
 * extension classes, that is, jar files placed into any of the usual extension
 * directories. Installed providers are loaded using the service-provider loading
 * facility defined by the {@link ServiceLoader} class. A ZoneRulesProvider
 * identifies itself with a provider configuration file named
 * {@code java.time.zone.ZoneRulesProvider} in the resource directory
 * {@code META-INF/services}. The file should contain a line that specifies the
 * fully qualified concrete zonerules-provider class name.
 * Providers may also be made available by adding them to the class path or by
 * registering themselves via {@link #registerProvider} method.
 * <p>
 * The Java virtual machine has a default provider that provides zone rules
 * for the time-zones defined by IANA Time Zone Database (TZDB). If the system
 * property {@code java.time.zone.DefaultZoneRulesProvider} is defined then
 * it is taken to be the fully-qualified name of a concrete ZoneRulesProvider
 * class to be loaded as the default provider, using the system class loader.
 * If this system property is not defined, a system-default provider will be
 * loaded to serve as the default provider.
 * <p>
 * Rules are looked up primarily by zone ID, as used by {@link ZoneId}.
 * Only zone region IDs may be used, zone offset IDs are not used here.
 * <p>
 * Time-zone rules are political, thus the data can change at any time.
 * Each provider will provide the latest rules for each zone ID, but they
 * may also provide the history of how the rules changed.
 *
 * @implSpec This interface is a service provider that can be called by multiple threads.
 * Implementations must be immutable and thread-safe. <p> Providers must ensure that once a rule has
 * been seen by the application, the rule must continue to be available. <p> Providers are
 * encouraged to implement a meaningful {@code toString} method. <p> Many systems would like to
 * update time-zone rules dynamically without stopping the JVM. When examined in detail, this is a
 * complex problem. Providers may choose to handle dynamic updates, however the default provider
 * does not.
 * @since 1.8
 */
public abstract class ZoneRulesProvider {

  /**
   * The set of loaded providers.
   */
  private static final CopyOnWriteArrayList<ZoneRulesProvider> PROVIDERS = new CopyOnWriteArrayList<>();
  /**
   * The lookup from zone ID to provider.
   */
  private static final ConcurrentMap<String, ZoneRulesProvider> ZONES = new ConcurrentHashMap<>(512,
      0.75f, 2);

  static {
    // if the property java.time.zone.DefaultZoneRulesProvider is
    // set then its value is the class name of the default provider
    final List<ZoneRulesProvider> loaded = new ArrayList<>();
    AccessController.doPrivileged(new PrivilegedAction<Object>() {
      public Object run() {
        String prop = System.getProperty("java.time.zone.DefaultZoneRulesProvider");
        if (prop != null) {
          try {
            Class<?> c = Class.forName(prop, true, ClassLoader.getSystemClassLoader());
            ZoneRulesProvider provider = ZoneRulesProvider.class.cast(c.newInstance());
            registerProvider(provider);
            loaded.add(provider);
          } catch (Exception x) {
            throw new Error(x);
          }
        } else {
          registerProvider(new TzdbZoneRulesProvider());
        }
        return null;
      }
    });

    ServiceLoader<ZoneRulesProvider> sl = ServiceLoader
        .load(ZoneRulesProvider.class, ClassLoader.getSystemClassLoader());
    Iterator<ZoneRulesProvider> it = sl.iterator();
    while (it.hasNext()) {
      ZoneRulesProvider provider;
      try {
        provider = it.next();
      } catch (ServiceConfigurationError ex) {
        if (ex.getCause() instanceof SecurityException) {
          continue;  // ignore the security exception, try the next provider
        }
        throw ex;
      }
      boolean found = false;
      for (ZoneRulesProvider p : loaded) {
        if (p.getClass() == provider.getClass()) {
          found = true;
        }
      }
      if (!found) {
        registerProvider0(provider);
        loaded.add(provider);
      }
    }
    // CopyOnWriteList could be slow if lots of providers and each added individually
    PROVIDERS.addAll(loaded);
  }

  //-------------------------------------------------------------------------

  /**
   * Gets the set of available zone IDs.
   * <p>
   * These IDs are the string form of a {@link ZoneId}.
   *
   * @return a modifiable copy of the set of zone IDs, not null
   */
  public static Set<String> getAvailableZoneIds() {
    return new HashSet<>(ZONES.keySet());
  }

  /**
   * Gets the rules for the zone ID.
   * <p>
   * This returns the latest available rules for the zone ID.
   * <p>
   * This method relies on time-zone data provider files that are configured.
   * These are loaded using a {@code ServiceLoader}.
   * <p>
   * The caching flag is designed to allow provider implementations to
   * prevent the rules being cached in {@code ZoneId}.
   * Under normal circumstances, the caching of zone rules is highly desirable
   * as it will provide greater performance. However, there is a use case where
   * the caching would not be desirable, see {@link #provideRules}.
   *
   * @param zoneId the zone ID as defined by {@code ZoneId}, not null
   * @param forCaching whether the rules are being queried for caching, true if the returned rules
   * will be cached by {@code ZoneId}, false if they will be returned to the user without being
   * cached in {@code ZoneId}
   * @return the rules, null if {@code forCaching} is true and this is a dynamic provider that wants
   * to prevent caching in {@code ZoneId}, otherwise not null
   * @throws ZoneRulesException if rules cannot be obtained for the zone ID
   */
  public static ZoneRules getRules(String zoneId, boolean forCaching) {
    Objects.requireNonNull(zoneId, "zoneId");
    return getProvider(zoneId).provideRules(zoneId, forCaching);
  }

  /**
   * Gets the history of rules for the zone ID.
   * <p>
   * Time-zones are defined by governments and change frequently.
   * This method allows applications to find the history of changes to the
   * rules for a single zone ID. The map is keyed by a string, which is the
   * version string associated with the rules.
   * <p>
   * The exact meaning and format of the version is provider specific.
   * The version must follow lexicographical order, thus the returned map will
   * be order from the oldest known rules to the newest available rules.
   * The default 'TZDB' group uses version numbering consisting of the year
   * followed by a letter, such as '2009e' or '2012f'.
   * <p>
   * Implementations must provide a result for each valid zone ID, however
   * they do not have to provide a history of rules.
   * Thus the map will always contain one element, and will only contain more
   * than one element if historical rule information is available.
   *
   * @param zoneId the zone ID as defined by {@code ZoneId}, not null
   * @return a modifiable copy of the history of the rules for the ID, sorted from oldest to newest,
   * not null
   * @throws ZoneRulesException if history cannot be obtained for the zone ID
   */
  public static NavigableMap<String, ZoneRules> getVersions(String zoneId) {
    Objects.requireNonNull(zoneId, "zoneId");
    return getProvider(zoneId).provideVersions(zoneId);
  }

  /**
   * Gets the provider for the zone ID.
   *
   * @param zoneId the zone ID as defined by {@code ZoneId}, not null
   * @return the provider, not null
   * @throws ZoneRulesException if the zone ID is unknown
   */
  private static ZoneRulesProvider getProvider(String zoneId) {
    ZoneRulesProvider provider = ZONES.get(zoneId);
    if (provider == null) {
      if (ZONES.isEmpty()) {
        throw new ZoneRulesException("No time-zone data files registered");
      }
      throw new ZoneRulesException("Unknown time-zone ID: " + zoneId);
    }
    return provider;
  }

  //-------------------------------------------------------------------------

  /**
   * Registers a zone rules provider.
   * <p>
   * This adds a new provider to those currently available.
   * A provider supplies rules for one or more zone IDs.
   * A provider cannot be registered if it supplies a zone ID that has already been
   * registered. See the notes on time-zone IDs in {@link ZoneId}, especially
   * the section on using the concept of a "group" to make IDs unique.
   * <p>
   * To ensure the integrity of time-zones already created, there is no way
   * to deregister providers.
   *
   * @param provider the provider to register, not null
   * @throws ZoneRulesException if a zone ID is already registered
   */
  public static void registerProvider(ZoneRulesProvider provider) {
    Objects.requireNonNull(provider, "provider");
    registerProvider0(provider);
    PROVIDERS.add(provider);
  }

  /**
   * Registers the provider.
   *
   * @param provider the provider to register, not null
   * @throws ZoneRulesException if unable to complete the registration
   */
  private static void registerProvider0(ZoneRulesProvider provider) {
    for (String zoneId : provider.provideZoneIds()) {
      Objects.requireNonNull(zoneId, "zoneId");
      ZoneRulesProvider old = ZONES.putIfAbsent(zoneId, provider);
      if (old != null) {
        throw new ZoneRulesException(
            "Unable to register zone as one already registered with that ID: " + zoneId +
                ", currently loading from provider: " + provider);
      }
    }
  }

  /**
   * Refreshes the rules from the underlying data provider.
   * <p>
   * This method allows an application to request that the providers check
   * for any updates to the provided rules.
   * After calling this method, the offset stored in any {@link ZonedDateTime}
   * may be invalid for the zone ID.
   * <p>
   * Dynamic update of rules is a complex problem and most applications
   * should not use this method or dynamic rules.
   * To achieve dynamic rules, a provider implementation will have to be written
   * as per the specification of this class.
   * In addition, instances of {@code ZoneRules} must not be cached in the
   * application as they will become stale. However, the boolean flag on
   * {@link #provideRules(String, boolean)} allows provider implementations
   * to control the caching of {@code ZoneId}, potentially ensuring that
   * all objects in the system see the new rules.
   * Note that there is likely to be a cost in performance of a dynamic rules
   * provider. Note also that no dynamic rules provider is in this specification.
   *
   * @return true if the rules were updated
   * @throws ZoneRulesException if an error occurs during the refresh
   */
  public static boolean refresh() {
    boolean changed = false;
    for (ZoneRulesProvider provider : PROVIDERS) {
      changed |= provider.provideRefresh();
    }
    return changed;
  }

  /**
   * Constructor.
   */
  protected ZoneRulesProvider() {
  }

  //-----------------------------------------------------------------------

  /**
   * SPI method to get the available zone IDs.
   * <p>
   * This obtains the IDs that this {@code ZoneRulesProvider} provides.
   * A provider should provide data for at least one zone ID.
   * <p>
   * The returned zone IDs remain available and valid for the lifetime of the application.
   * A dynamic provider may increase the set of IDs as more data becomes available.
   *
   * @return the set of zone IDs being provided, not null
   * @throws ZoneRulesException if a problem occurs while providing the IDs
   */
  protected abstract Set<String> provideZoneIds();

  /**
   * SPI method to get the rules for the zone ID.
   * <p>
   * This loads the rules for the specified zone ID.
   * The provider implementation must validate that the zone ID is valid and
   * available, throwing a {@code ZoneRulesException} if it is not.
   * The result of the method in the valid case depends on the caching flag.
   * <p>
   * If the provider implementation is not dynamic, then the result of the
   * method must be the non-null set of rules selected by the ID.
   * <p>
   * If the provider implementation is dynamic, then the flag gives the option
   * of preventing the returned rules from being cached in {@link ZoneId}.
   * When the flag is true, the provider is permitted to return null, where
   * null will prevent the rules from being cached in {@code ZoneId}.
   * When the flag is false, the provider must return non-null rules.
   *
   * @param zoneId the zone ID as defined by {@code ZoneId}, not null
   * @param forCaching whether the rules are being queried for caching, true if the returned rules
   * will be cached by {@code ZoneId}, false if they will be returned to the user without being
   * cached in {@code ZoneId}
   * @return the rules, null if {@code forCaching} is true and this is a dynamic provider that wants
   * to prevent caching in {@code ZoneId}, otherwise not null
   * @throws ZoneRulesException if rules cannot be obtained for the zone ID
   */
  protected abstract ZoneRules provideRules(String zoneId, boolean forCaching);

  /**
   * SPI method to get the history of rules for the zone ID.
   * <p>
   * This returns a map of historical rules keyed by a version string.
   * The exact meaning and format of the version is provider specific.
   * The version must follow lexicographical order, thus the returned map will
   * be order from the oldest known rules to the newest available rules.
   * The default 'TZDB' group uses version numbering consisting of the year
   * followed by a letter, such as '2009e' or '2012f'.
   * <p>
   * Implementations must provide a result for each valid zone ID, however
   * they do not have to provide a history of rules.
   * Thus the map will contain at least one element, and will only contain
   * more than one element if historical rule information is available.
   * <p>
   * The returned versions remain available and valid for the lifetime of the application.
   * A dynamic provider may increase the set of versions as more data becomes available.
   *
   * @param zoneId the zone ID as defined by {@code ZoneId}, not null
   * @return a modifiable copy of the history of the rules for the ID, sorted from oldest to newest,
   * not null
   * @throws ZoneRulesException if history cannot be obtained for the zone ID
   */
  protected abstract NavigableMap<String, ZoneRules> provideVersions(String zoneId);

  /**
   * SPI method to refresh the rules from the underlying data provider.
   * <p>
   * This method provides the opportunity for a provider to dynamically
   * recheck the underlying data provider to find the latest rules.
   * This could be used to load new rules without stopping the JVM.
   * Dynamic behavior is entirely optional and most providers do not support it.
   * <p>
   * This implementation returns false.
   *
   * @return true if the rules were updated
   * @throws ZoneRulesException if an error occurs during the refresh
   */
  protected boolean provideRefresh() {
    return false;
  }

}
